Leifer, Stephanie D. (1995) Characterization of fullerenes for electrostatic propulsion applications. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10152007-152404
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
[...], also known as Buckminsterfullerene, possesses a remarkable resilience, high mass, and low ionization potential which indicate that it could make an excellent ion engine propellant. The development of an efficient, reliable [...] ion thruster for space flight requires a knowledge of many of the properties of this new molecule. The research described here focusses on the determination of some of these properties and their effects on the behavior of [...] in a plasma environment. Investigations of fullerene thermal stability, polarizability, electron-impact ionization and fragmentation phenomena, vibrational spectra, and plasma discharges are reported.
The electron-impact ionization and fragmentation experiments were conducted using a time-of-flight mass spectrometer. Ionization efficiency curves for the production of both [...] and [...] ions were obtained, and appearance potentials of [...] eV for [...] and [...]eV for [...] and [...], respectively were identifed. Fullerene cracking patterns showed only even-numbered fragments, and only at electron energies above 70 eV. Multiply charged fullerene ions through [...] were observed, revealing the remarkable resistance of [...] to coulomb explosion.
Experiments to determine the polarizability of [...] using a Mach-Zehnder interferometer and the Clausius-Mossotti relation were performed. The data yielded a polarizability value near [...].
Although previous experiments performed by other researchers with filament cathode discharge chambers successfully demonstrated the production of [...] plasma at a minimum discharge voltage of 22 V, difficulty with severe erosion of the tungsten filament cathode was encountered. Also, decomposition of the fullerenes at temperatures above 1073 K was observed.
In light of these observations, the thermal stability of a fullerene mix was examined. Decay constants were obtained and used to find an Arrhenius plot for the thermal decomposition. The Arrhenius activation energy was found to be [...] kJ/mol. The disintegration of [...] occurs at significantly lower temperatures than those predicted by molecular dynamics simulations. A mechanism for fullerene disintegration different than [...] elimination, possibly involving ring-rearrangement in the fullerene cage, may be responsible.
Because the determination of the purity of fullerene samples is vital when investigating their degradation at elevated temperatures, a study of fullerene contaminants and adsorbates using Fourier Transform Infrared (FTIR) spectroscopy was conducted. For solid fullerene samples, Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy was employed. Features in the 2350, 2330, and 1540 [...] region of the spectrum were found to be intrinisic to [...] and did not result from CO2 or O2 contamination. By increasing the CO2 content of solid [...], new features could be observed at 2377, 2330 and 2316 [...]. The peak observed at 1539 [...] in the solid vanished when fullerenes were placed in solution, but reappeared when the sample was dried under N2. The strength of this peak may be due to a Fermi resonance or crystal field effect, both of which could be destroyed by solvent interactions in solution.
To avoid the presence of high temperature metal surfaces, a RF discharge chamber was constructed to maintain a fullerene plasma inductively, eliminating the presence of hot electrode surfaces. A xenon plasma was successfully generated in the thruster. The maximum extractable beam current was approximately 45 mA. Ion production cost for a pure xenon plasma was 1400 eV/ion. When [...] vapor was added to the xenon plasma, the discharge quenched when the ratio of [...] molecules to xenon atoms exceeded 1:16. A pure fullerene RF discharge could not be initiated.
The cause of this phenomenon can be found in the electron attachment cross section of [...] this cross section is very large for electron energies up to 14 eV. It would be necessary to maintain an electron temperature of 10 eV or greater for positive fullerene ion production rates to exceed that of negative ion production rates.
In summary, this research has been an investigation of some fundamental properties of [...] relevant to fullerene ion thruster development. It was found that although fragmentation by electron impact of [...] does not pose a significant problem, the thermal stability of fullerenes is not as great as had been anticipated. As a result, use of conventional hollow cathode configurations in fullerene ion thrusters is precluded. A RF fullerene ion thruster configuration was pursued instead, but proved difficult to operate because electron attachment cross sections for [...] remain large up to 14 eV electron energy. Therefore, the use of other ionization mechanisms may prove more successful for fullerene ion thruster development, and should be pursued in future work.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Applied Physics|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||26 May 1995|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||26 Oct 2007|
|Last Modified:||26 Dec 2012 03:05|
- Final Version
Restricted to Caltech community only
See Usage Policy.
Repository Staff Only: item control page